The travel booking site Kayak added price forecasting to its flight booking site today, which should help you decide when the best time to book a cheap flight is.

Kayak's already one of your favorite cheap travel booking sites, and the addition of price forecasting makes it more useful. The forecasting is based on Kayak's analysis of prices over the last year. When you're booking a flight, simply look to the left of your flight list, and click the "Price Trend" button. There you're given the fare trends for the dates you selected, a recommendation on whether you should buy now or not, as well as a list of when flights might be cheaper. The feature is live right now, so head over to Kayak to see how it works.